The invention relates generally to frequency oscillators, and more particularly the invention relates to the use of temperature dependent resonators to establish oscillation frequency.
It has been shown that RF surface micro-machined MEMS resonators were potential replacement part for quartz resonators in reference oscillator applications. The main advantage relies in the form factor and path to on-chip integration, but this is at the expense of accuracy of the resonance frequency and higher temperature drift.
Indeed for wireless handset application, state of the art quartz resonators exhibits frequency tolerance of +/−2 ppm after hand trimming involving deposition of infinitesimal quantity of metal at the quartz surface. For the same application, it has become clear that laterally vibrating bulk resonators were the structures of choice to fulfill application requirements in term of frequency, quality factor and power handling. But, in the case of surface micro-machined MEMS resonators, one can evaluate to the first order the fabrication tolerance on the absolute resonance frequency using tolerance on the lateral dimensions. State of the art lithography tools can achieve +/−15 nm in tolerance. For GSM/CDMA, typical resonator dimensions are in the range of 30–60 μm. This translates into an absolute tolerance of higher than 200 ppm. This already high value compared to quartz is assumed without taking into account effect of the stress in the package and other non-idealities in the micromachining process (anchors, alignment between layers, anisotropy . . . ) and makes a difficult trimming of the frequency necessary.
In terms of thermal characteristic, research has shown that surface micro-machined resonators (polysilicon, silicon-germanium, or piezoelectric) exhibit a temperature drift of more than 2500 ppm over a −30° C.–85° C. range. This makes the use of compensation techniques developed for quartz crystal oscillators very difficult, where a typical AT-cut first mode resonator experiences a maximum excursion of +/−20 ppm over the full temperature range.